libexec/rc: Add var_run rc script

Users with a tmpfs /var/run will lose the directory tree state of
/var/run at reboot. This rc script will optionally (by default)
capture the state of the directory structure in /var/run prior to
shutdown and recreate it at system boot.

Alternatively a user can save the state of the /var/run directories
manually using service var_run save and disable the autosaving of
/var/run state using the var_run_autosave variable, for those
paranoid SSD users.

PR: 259585, 259699
Reported by: freebsd@walstatt-de.de,
Reviewed by: philip, gbe (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36386